The present invention relates to plasma processing.
The fabrication of modern semiconductor device structures has traditionally relied on plasma processing in a variety of operations such as etching, depositing or sputtering. Plasma etching involves using chemically active atoms or energetic ions to remove material from a substrate. Plasma Enhanced Chemical Vapor Deposition (PECVD) uses plasma to dissociate and activate chemical gas so that the substrate temperature can be reduced during deposition. Plasma sputtering also deposits materials onto substrates, where plasma ions such as argon impact a material surface and sputter the material that is then transported as neutral atoms to a substrate. Additional plasma processes include plasma surface cleaning and physical-vapor deposition (PVD) of various material layers.
Conventionally, plasma is generated using a radio frequency powered plasma source. In a “typical” radio frequency powered plasma source, alternating current (AC) power is rectified and switched to provide current to an RF amplifier. The RF amplifier operates at a reference frequency (13.56 MHz, for example), drives current through an output-matching network, and then through a power measurement circuit to the output of the power supply. The output match is usually designed to be connected a generator that is optimized to drive particular impedance such as 50 ohms, in order to have the same characteristic impedance as the coaxial cables commonly used in the industry. Power flows through the matched cable sections, is measured by the match controller, and is transformed through the load match. The load match is usually a motorized automatic tuner, so the load match operation incurs a predetermined time delay before the system is properly configured. After passing through the load match, power is then channeled into a plasma excitation circuit that drives two electrodes in an evacuated processing chamber. A processing gas is introduced into the evacuated processing chamber, and when driven by the circuit, plasma is generated. Since the matching network or the load match is motorized, the response time from the matching network is typically in the order of one second or more.
Conventionally, plasma is continuously generated in order to obtain the large amount of power necessary to deposit the layers at high speed and thereby to improve the shapes of stepped parts thereof (coverage). As noted in U.S. Pat. No. 5,468,341 entitled “Plasma-etching method and apparatus therefor”, the amount of ion energy reaching a surface of the object to be etched in conventional RF sources can be accomplished by controlling the power of RF waves, the controllable range of dissociation process in plasmas is narrow and, therefore, the extent of controllable etching reactions on the surface of the object wafer is narrowly limited. Also, since the magnetic fields are present in a plasma generation chamber for high-density plasmas, a magnetohydrodynamic plasma instability can exist due to, for example, drift waves generated in the plasmas, which leads to a problem wherein the ion temperature rises and the directions of ion motions become nonuniform. Further, the problems include a degradation of a gate oxide film and a distortion of etching profile due to the charges accumulated on the wafer.
In a deposition technology known as atomic layer deposition (ALD), various gases are injected into the chamber for as short as 100-500 milliseconds in alternating sequences. For example, a first gas is delivered into the chamber for about 500 milliseconds and the substrate is heated, then the first gas (heat optional) is turned off. Another gas is delivered into the chamber for another 500 milliseconds (heat optional) before the gas is turned off. The next gas is delivered for about 500 milliseconds (and optionally heated) before it is turned off. This sequence is done for until all gases have been cycled through the chamber, each gas sequence forming a mono-layer which is highly conformal. ALD technology thus pulses gas injection and heating sequences that are between 100 and 500 milliseconds. This approach has a high dissociation energy requirement to break the bonds in the various precursor gases such as silane and oxygen and thus requires the substrate to be heated to a high temperature, for example in the order of 600-800 degree Celsius for silane and oxygen processes
Conventional ALD equipment typically uses coils with the same width to height aspect ratio. The plasma density for the conventional equipment is focused at the coil. The density is high near the coil, and typically the density becomes uniform at about five inches from the coil. Such non-linearity in plasma density is undesirable.